Magnetic tape and its recent companion, optical tape, are used to store large amounts of data (both digital and analog) at a cost that is significantly less than other competing forms of mass storage, such as magnetic floppy disks, hard disk drives, or write once, read many (WORM) drives. Perhaps as a result, tape drives are often used to backup (i.e., to copy one or more files or programs for archival purposes) the other forms of mass storage. The backup tapes are conveniently packaged in cartridges of standard dimensions, such as VHS and DAT (digital audio tape). The cartridges protect the tape from dirt and other contaminants, store the tape in a convenient form, and allow a user to readily change the tapes in a tape drive without the need to thread the tape onto a reel.
Besides backing up other forms of mass storage, tape has not found widespread use as a medium for storing multiple files of data. The other forms of mass storage are preferred over tape storage because, among other reasons, tape storage requires significantly more time to access data. With tape, accessing a particular file may require forwarding (or rewinding) through most of the tape, a linear process. In contrast, with a optical or magnetic disk drive the reading head can access a particular disk location by moving in two dimensions, either spiraling along a track, or moving radially to cut across tracks.
At present as a prelude to accessing data on a tape, most tape drives rewind a tape when the tape is initially inserted into the drive. (The beginning or end of tape in most cartridge tapes is indicated by a material that can be sensed by the tape drive, a provision that simplifies rewinding a tape.) Knowing what track the particular file begins on, the drive could access the file by moving the tape at a normal speed (e.g., 0.32 inches per second (ips) for a DAT) past a transducer which is typically a rotating helical scan head. The head would scan over each track, reading the number of the sequentially numbered tracks (the track number being part of the overhead associated with the data of each track). When the drive reached the appropriately numbered track, the drive would begin reading data.
In reality, a faster method is used to locate the beginning track of a particular file. The tape drive first locates the approximate position of the beginning track by performing a "fast forward" read. With the fast forward read, the tape moves past the transducer head at about 60 ips, with little change in the scanning speed of the rotation head. Consequently the head does not read the track number of every track. Upon reaching the vicinity of the starting track of a file, the drive resumes a normal reading of track numbers so that the beginning track of the file can be precisely located.
While the above method is generally adequate, there is a need for a faster method for determining the initial position of a tape in a tape cartridge.